Immortality

This mod adds a potion that prevents you from dying while under its effects. Specifically, it prevents your health from dropping below about 20% (multiplied by the effect level as of 1.5), rounded up. It is brewed with the philosopher's stone, an item that generates in soul sand patches in the nether. The ore is affected by fortune, and is mined with a shovel. The potion comes in regular, splash, lingering and tipped arrow variants, and can be extended with redstone as usual. The default potion lasts for 10 seconds, and the extended lasts for 20 seconds.

Here's a demonstration of immortality in action:

Modpack policy: Unless I say no, go ahead! Just be sure to link back here of course!